1. Field of the Invention
The present invention relates to an NMR probe operative to detect nuclear magnetic resonance (NMR) signals and, more particularly, to a cooled NMR detection probe structure for detecting NMR signals.
2. Description of Related Art
(1) Fundamental Configuration of NMR Instrument
An NMR instrument is an apparatus for analyzing the molecular structure of a sample under investigation by placing it in a static magnetic field, applying an RF signal to the sample, then detecting a feeble RF signal (NMR signal) emanating from the sample, and extracting molecular structure information contained in the detected signal.
The positional relationship between a conventional NMR probe and a superconducting magnet producing a static magnetic field is shown in FIG. 20. The superconducting magnet, indicated by A, has windings of superconducting wire therein to form a main wound coil B. The main coil B is normally placed in an adiabatic vessel (not shown) capable of storing liquid helium or the like, and is cryogenically cooled. The NMR probe, indicated by C, is configured including a jawlike base portion 400 disposed outside the magnet and a tubular portion 410 inserted inside the magnet. The tubular portion 410 is provided within a cylindrical through-hole D extending along the central axis of the superconducting magnet A. Usually, the cylindrical portion 410 is inserted upward into the hole D from the opening located below the hole D.
FIG. 21 schematically shows the configuration of the NMR instrument. This instrument has an RF oscillator 501 generating an RF signal which is controlled in phase and amplitude by a phase controller 502 and an amplitude controller 503. The signal is fed to a power amplifier 504, where the RF signal is amplified to a power necessary to excite an NMR signal. The RF signal is then fed to an NMR probe 506 via a duplexer 505. The signal is then applied to the sample under investigation from a detection coil (not shown) placed in the NMR probe 506.
After the RF irradiation, a feeble NMR signal generated from the sample under investigation is detected by the detection coil (not shown) placed in the NMR probe 506 and again passed through the duplexer 505. The signal is then fed to a preamplifier 507, where the signal is amplified to a signal intensity permitting reception.
A receiver 508 converts the RF NMR signal amplified by the preamplifier 507 to an audio frequency that can be converted into digital form. At the same time, the receiver controls the amplitude. The audio-frequency NMR signal from the receiver 508 is converted into digital form by an analog-to-digital data converter 509 and sent to a control computer 510. The control computer 510 controls the phase controller 502 and the amplitude controller 503 and Fourier-transforms the NMR signal accepted in the time domain. The computer automatically corrects the phase of the Fourier-transformed NMR signal and displays the signal as an NMR spectrum.
(2) General Explanation of the Prior Art
A cooled NMR detection probe (known as a cryoprobe) is known as one type of NMR probe. This probe utilizes a vacuum vessel such that various parts (especially, components of a detection system) in the vacuum vessel are placed in a cryogenetic state. The especially important part of the cooled components is the detection module (hereinafter may be referred to as the core module) for detecting NMR signals. For example, a conventional core module is composed of a bobbin and a coil wound around the outer surface of the bobbin. A heat exchanger is disposed in the vacuum vessel to cool the detection system. If the sample under investigation is a solid, the sample tube holding the solid sample therein is rotatably arranged in a posture tilted at a given angle (so-called the magic angle) within the cylindrical partition wall of the vacuum vessel. During investigation of the sample, the sample tube is spun at high speed. At this time, the sample itself is placed at room temperature. On the other hand, the detection system (especially, the core module) in the vacuum vessel is placed at a cryogenic temperature as described previously. Measurements may be performed at arbitrary sample temperatures. Cooled components existing in the vacuum vessel include the aforementioned core module, elements constituting the detection circuit (variable capacitor, fixed capacitor, and so on), a duplexer for switching the signal between outgoing signal and receiving signal, a preamplifier, a directional coupler, a coaxial cable, and a radiation shield.
(3) Sensitivity of NMR Probe
NMR signals observed using an NMR probe have frequencies lying in the range from several MHz to hundreds of MHz. In this frequency range, principal noise is Johnson noise (thermal noise) caused by fluctuations of phonons within a conductor constituting the signal detection circuit. The intensity of such thermal noise (in voltage) is constant irrespective of the frequency (carrier frequency) of the sent signal. Thus, so-called white noise is generated. Other noises include flicker noise which can be neglected at radio frequencies. In order to improve the sensitivity by reducing thermal noise, it is desired to cool the detection system. The cooled NMR detection probe has been developed under this concept, and achieves high detection sensitivity.
In NMR spectral measurements, signal-to-noise ratio (S/N) is generally given by the following equation:
                                          V            S                                V            N                          =                              M            0                    ⁢          sin          ⁢                                          ⁢                      θ            m                    ⁢                                                                      Q                  c                                ⁢                                  μ                  0                                ⁢                                  η                  f                                ⁢                                  v                  s                                ⁢                                  ω                  0                                                            4                ⁢                                                      k                    B                                    ⁡                                      (                                                                  T                        c                                            +                                              T                        a                                                              )                                                  ⁢                Δ                ⁢                                                                  ⁢                f                                                                        (        1        )            Note that dielectric coupling is neglected.
The parameters in the above equation have the following meanings. VS is an electromotive force (in voltage) inducing an NMR signal attributed to nuclear spins. VN is the sum of electromotive forces (in voltage) produced by noises generated from the transmit/receive coil to the preamplifier output. M0 indicates thermally balanced magnetization of nuclear spins in a static magnetic field. θm is an angle made between the oscillating magnetic field produced by the transmit/receive coil and the static magnetic field. In the case of NMR spectroscopy for a solid sample, this angle is the magic angle. In particular, this angle satisfies the relationship:
      cos    ⁢                  ⁢          θ      m        =            1      /              3              ⁢          (              roughly        ⁢                                  ⁢        54.7        ⁢        °            )      
μ0 is the vacuum permeability. vs is the volume of the sample. ω0 is the Larmor frequency of nuclear spins in the static magnetic field. kB is the Boltzmann constant. Δf is the observation bandwidth. Qc is the Q's value of the transmit/receive coil. ηf is the filling factor, which is, in principle, the ratio between the magnetic field strength in the sample space and the whole magnetic field strength felt by the transmit/receive coil. Where the transmit/receive coil is a solenoid coil, the filling factor can be simply represented as the ratio of the sample volume to the cylindrical volume inside the coil. Tc is the temperature of the conductor constituting the transmit/receive coil. Ta is the temperature (noise temperature) of the preamplifier.
The cooled NMR detection probe reduces the thermal noise (depending on Tc) arising from the conductor forming the transmit/receive coil by placing the sample at room temperature or at a given temperature, spinning the sample at high speed, and placing the detector coil (acting also as an RF pulse transmitter coil; transmit/receive coil in this meaning) at a cryogenic temperature (e.g., below 20 K). Also, the Q value (Qc) of the transmit/receive coil is improved by lowering the resistance (depending on Tc) of the conductor forming the transmit/receive coil, i.e., by lowering the RF resistance.
However, in the cooled NMR detection probe, it is necessary to form a cylindrical partition wall to create a vacuum insulating layer between the transmit/receive coil and the sample tube. Therefore, the inside diameter of the transmit/receive coil is inevitably greater than that of an ordinary NMR probe. This makes the filling factor smaller than that of an ordinary NMR probe (see FIG. 3 that will be referenced later). To address this problem, it is desired that the inside diameter of the transmit/receive coil be reduced to increase the filling factor. If the wall thickness of the bobbin around which the transmit/receive coil is wound is reduced for that purpose, then it will be impossible to sufficiently cool the transmit/receive coil via the bobbin (i.e., Tc will vary). In this way, restrictions are imposed on thinning of the bobbin wall. That is, there is a restriction on reduction of the inside diameter of the transmit/receive coil.
(4) Detailed Explanation of the Prior Art
Referring to FIGS. 1-4, there is shown a cooled NMR detection probe (herein referred to as the comparative example). This probe is to be compared with an embodiment described later (see JP-A-2008-241493). This probe has an insertion portion 14 that is inserted into a bore 12 formed in a magnetic field generator 10. The insertion portion 14 is composed of a probe head 16 and a probe body 18. A vacuum vessel 20 forms a partition wall. A sleeve 24 acting as a cylindrical partition wall is formed in an upper part of the vacuum vessel 20. The sleeve 24 has an internal passage in which a sample tube 26 is rotatably inserted. The sample tube 26 is disposed in a posture tilted at a given angle.
A rotating mechanism 28 gives a rotating force to the opposite ends of the sample tube 26 while holding them. The inner passage of the sleeve 24 is at atmospheric pressure and room temperature. The inside 22 of the vessel 20 is in a vacuum state. The parts placed in the vessel 20 are at low temperatures. A core module 30 forming a main part of the detection system is disposed in the probe head 16. In this example, the core module 30 includes a transmit/receive coil 34 and a bobbin 32 shaped like a silk hat and having a cylindrical portion 32A. The transmit/receive coil 34 is solenoidal in shape and formed on the outer surface of the cylindrical portion 32A of the bobbin 32. The coil 34 applies an RF magnetic field to the sample to induce a nuclear magnetic resonance, which is detected as a signal by the coil 34. The bobbin 32 has a jawlike portion (circular fringes) 32B to which an upper-end portion of a flexible heat link 38 is coupled. The bobbin 32 is made, for example, of sapphire. In the illustrated example, the bobbin 32 is fixed to the vacuum vessel 20 via a plurality of jigs (not shown) made of a heat insulating material such as FRP (fiber-reinforced plastic). The lower end of the heat link 38 is coupled to one end of a heat exchanger 36. Liquid helium, for example, is introduced in the heat exchanger 36. As a result, the heat exchanger is placed at a cryogenic temperature (e.g., 4 K) chiefly by latent heat produced by evaporation of helium. The heat exchanger 36 is in thermal communication with the bobbin 32 via the heat link 38. Consequently, the transmit/receive coil 34 is cooled.
FIG. 2 is a perspective view of the core module 30 shown in FIG. 1. The core module 30 includes the bobbin 32 shaped like a silk hat and the transmit/receive coil 34 (not shown in FIG. 2) as described previously. The sleeve 24 forming a part of the vacuum vessel is disposed in the cylindrical portion of the bobbin 32. The sample tube 26 is inserted in the sleeve 24. The flexible heat link 38 consisting of belt-like members 38A and 38B is formed across the surface of the jawlike portion of the bobbin 32 and the heat exchanger 36. The heat link 38 acts as a heat conducting member. The belt-like members 38A and 38B are in loose state. As described in detail later, if the heat exchanger 36 is shifted downward due to contraction during cooling, the distance between the bobbin 32 fixed to the vacuum vessel and the displaced heat exchanger 36 varies. The variation in the distance is absorbed by slackening of the belt-like members. In other words, if the heat exchanger 36 is shifted downward due to contraction, the bobbin 32 is securely fixed to the vacuum vessel to prevent the position of the core module 30 from varying. In FIG. 3, the diameters of the members are denoted. That is, the sample tube 26 has an inside diameter of φ1 and an outside diameter of φ2. The sleeve 24 has an inside diameter of φ3 and an outside diameter of φ4. The cylindrical portion 32A of the bobbin has an inside diameter of φ5 and an outside diameter of φ6. Preferably, the transmit/receive coil is placed as close as possible to the sample but the sleeve 24 and bobbin cylindrical portion 32A exist between them. Therefore, there is a limitation on reducing the distance.
FIG. 4A schematically shows the cooled NMR detection probe under the condition in which the probe is not yet cooled but at room temperature. FIG. 4B schematically shows the cooled NMR detection probe under the condition in which the probe is being cooled. The heat exchanger 36 is mounted in the vacuum vessel 20 and supported by a pillar 37 having a base end securely fixed to a lower unit 43. A cylindrical radiation shield 39 reflects infrared radiation coming from the outside and infrared radiation emitted inwardly from the vacuum vessel 20. A connecting member 41, which is fixedly connected to the pillar 37, holds the radiation shield 39 to the pillar 37. The temperature of the vacuum vessel 20 itself is essentially at room temperature irrespective of whether the probe is in the state of FIG. 4A or in the state of FIG. 4B. On the other hand, the internal structure of the vacuum vessel 20 contracts as indicated by the arrows in FIG. 4B according to the linear expansion coefficients intrinsic to the various members during a transitional process from room temperature to cooled state. The various members are shifted downward toward the lower unit 43. A member located at a higher position experiences a larger amount of displacement. A reference height is indicated by h1. The height of the upper surface of the heat exchanger 36 is indicated by h2. The height of the detection center (sample center) is indicated by h3. Because of contraction of the various members, the height h2 of the upper surface of the heat exchanger 36 has decreased by Δh to a height h4. Concomitantly, the flexible heat sink 38 has elongated as indicated by 38a. Note that the elongation is shown exaggeratedly in FIG. 4B. On the other hand, the bobbin is securely held to the vacuum vessel 20 by the jigs (not shown) and so the height of the transmit/receive coil does not vary before and after the cooling.
(5) Consideration from a Point of View of Filling Factor
The filling factor is defined to be the ratio of the space occupied by the sample under investigation to the space of the RF magnetic field set up by the transmit/receive coil. The filling factor is a dimensionless number and has a positive proportional relationship to the NMR detection sensitivity performance (S/N). To a quite simple approximation, the ratio of the volume of the sample to the volume of the transmit/receive coil can be regarded as the filling factor. In NMR measurements on solid samples, the samples rarely extend beyond the volume of the transmit/receive coil. Assuming that the sample tube has a coaxial configuration, the filling factor can be approximated by the ratio of the outside diameter of the cylindrical sample to the inside diameter of the coil. Accordingly, from a point of view of S/N, the inside diameter of the coil is preferably set closer to the outside diameter of the sample.
In the above-described comparative example (or the conventional example), a solenoidal coil is formed on the outer surface of the cylindrical portion of the bobbin shaped like a silk hat. The inside diameter of the transmit/receive coil of this design is stipulated as follows (see also FIG. 3).[Inside diameter φ6 of transmit/receive coil]=[outside diameter of sample]+[thickness of sample tube]×2+[clearance of space in which sample spins]×2+[thickness of vacuum partition sleeve]×2+[clearance of vacuum insulating space]×2+[thickness of cylindrical portion of sapphire bobbin]×2   (2)
Optimization of the filling factor is to reduce the dimensions of the various items to bare minimum. However, when the final term, [thickness of cylindrical portion of sapphire bobbin], is noticed, if this thickness is reduced, the cross-sectional area of heat transfer of the bobbin decreases. This yields a result contrary to the requirement that the coil temperature should be made as low as possible.
Generally, an alternating magnetic field produced by an RF electric current is distributed along the surface of a conductor and does not readily penetrate into the interior of the conductor. The current density is also concentrated in the surface of the conductor (referred to as a surface effect). In addition, in the case of a conductor having a shape like a solenoid coil, each line element is affected by the alternating magnetic fields produced by adjacent line elements. As a result, the electric current is concentrated in the inner surface of the conductor (surface on the radially central side) and the current density is enhanced in this region (known as a proximity effect). Accordingly, in the conventional silk hat-shaped bobbin, the resistivity at the innermost surface (surface layer) of the conductor constituting the transmit/receive coil and the characteristics of the surface determine the magnitude of the loss. Where the transmit/receive coil is plated, particulates of a catalyst may be buried at the boundary between the metalized innermost surface of the coil conductor and the metalized basic material (sapphire) of the bobbin to enhance the adhesion between them. In this case, the electrical conductivity would be lower than where a bulk conductor such as oxygen-free copper is used. Also, the surface roughness would be greater. Where such a transmit/receive coil is supplied with an RF signal, the loss produced in this coil tends to be greater than the loss produced in a coil made of ideal bulk oxygen-free copper. This will deteriorate the coil's Q. If the coil temperature is lowered by cooling, the resistivity of the metal decreases but the concentration of the current density toward the interior of the coil is further enhanced. Hence, the circumstance that is disadvantageous from a point of view of the coil's Q is not improved by cooling.
The NMR probe disclosed in JP-UM-A-58-99655 (Japanese Utility Model Application Ser. No. 56-198075) has an insert coil. It is observed that a cylindrical member outwardly surrounds the insert coil. In this structure, the temperature of the insert coil is directly controlled by gas and so the cylindrical member does not function as a heat conducting medium. JP-UM-A-60-70083 (Japanese Utility Model Application Ser. No. 58-162897) sets forth a method of fabricating an NMR solenoid coil. The method consists of forming a coil pattern on a printed circuit board. Then, the solenoid coil is completed by rounding the printed circuit board while placing the coil surface on the inner side. In this structure, too, the printed circuit board does not function as a heat conducting medium.
(6) Consideration to Contraction Caused by Cooling
As shown in FIGS. 4A and 4B, during a transitional process from room-temperature state to cooled state, various members contract and thus are displaced downwardly. In the structure shown in FIGS. 4A and 4B, the height of the fixed end of the pillar 37 forms a reference height surface. Various members including the pillar 37 are displaced toward the reference height surface. Therefore, a member located at a higher position experiences a larger amount of displacement. If the core module 30 were connected only to the heat exchanger 36, the core module 30 would be displaced downwardly in cooled state. As a result, the bobbin would come into contact with the sleeve. This would ruin the adiabatic state of the core module 30. The cooled state of the transmit/receive coil would no longer be maintained. The temperature of the sample container would vary, resulting in the problem of formation of dew and frost. Accordingly, in the above-described comparative example, the bobbin is fixed to the vacuum vessel via the holding member. To secure heat conduction if the distance between the core module 30 and the heat exchanger 36 varies, the flexible heat link 38 made of foil or wire of a soft metal such as copper or silver is formed between the core module and the heat exchanger.
However, the thermal cross section of the heat link is relatively small and thus it is difficult to secure a sufficient amount of heat transfer through the link. For example, in the structure using the heat link 38 shown in FIGS. 4A and 4B, a temperature difference of 10 K is generated between the heat exchanger 36 and the bobbin. This creates the danger that the transmit/receive coil will not be cooled efficiently. The heat link 38 is made of a metal conductor and located close to the transmit/receive coil. Therefore, the characteristics of the transmit/receive coil might be adversely affected by the heat link 38. Although the jigs for holding the bobbin are made of a heat insulating material such as FRP, heat transfer through the jigs between a room-temperature body and a cryogenic body is not negligible. Therefore, use of such jigs should be minimised. Furthermore, from a point of view of thermal conductivity, the number of parts in the heat transfer path is preferably reduced.
The above-stated issues of contraction are also set forth in JP-A-2004-233337. In the structure shown in JP-A-2004-233337, an internal mechanism consisting of a cooler and a detector is disposed in a vacuum vessel. A pillar is mounted below the detector. The internal mechanism is supported from its underside by the pillar. On the other hand, the detector is fixedly mounted to the underside of the top plate of the vacuum vessel via a heat insulating member. Thus, the internal mechanism is supported also from its upper side. The heating insulating member is in the form of a flat plate such as a spacer. The upper surface of the heat insulating member is in intimate contact with the lower surface of the top plate. This lower surface is in intimate contact with the top surface of the detector. Regarding this structure, it can be pointed out that downward displacement of the detector due to contraction can be restricted but some amount of heat may flow in via the heat insulating member. It is intrinsically difficult to apply this structure to a cooled NMR detection probe for use with a solid sample.
In the conventional cooled type NMR detection probe, when the internal structure is secured to the lower unit, during a transitional process from room-temperature state to cooled state, as various members contract, the internal structure is displaced downward and, therefore, it has been urged to secure the core module (especially, the transmit/receive coil) to the vacuum vessel by jigs (securing members). This has presented the following problems.
First, there is the problem that heat flows in between the vacuum vessel at room temperature and the transmit/receive coil at a cryogenic temperature via the jigs. The inflow of heat creates the possibility that the transmit/receive coil cannot be cooled sufficiently.
Secondly, there is the problem that a heat link (heat conducting member) acting as a rigid body cannot be placed between the heat exchanger and the core module because the core module is kept fixed while the heat exchanger is displaced downward. In the conventional technique, flexible heat links have been used. Such heat links cannot provide good heat conduction. As a result, the core module may not be cooled sufficiently.